Water heaters are commonly used in homes, businesses and just about any establishment that has a need for heated water. Water heaters often heat water using the simple “heat rises” principle. In operation, water heaters typically heat cold or ambient temperature water entering at or near the bottom of the water heater tank to a desired temperature using a gas-fired burner, an electric heater or some other heater element. During a heating cycle, the cold or ambient temperature water at or near the bottom of the water heater tank becomes hotter and begins to rise towards the top of the water heater tank. Cooler and denser water, once on top of the water being heated, falls toward the bottom of the water heater tank so that it can be heated or reheated to the desired temperature. After the temperature of the water at the bottom of the water heater tank reaches a certain desired temperature, the water heater typically stops heating the water for a period of time.
A conventional water heater typically has at least one heating element or “heater,” such as a gas-fired and/or electric burner. To take advantage of the “heat-rises” principle, the heater is typically located at or near the bottom of the water heater tank. A sensor is often provided at or near the bottom end of the water heater tank to sense the temperature of the water. A thermostat or controller is often coupled to the sensor and the heater, and is programmed to control the heater to a water temperature set point based on the sensed temperature.
During operation, when demand for hot water arises (e.g., someone turns on a faucet), fresh, cold or ambient temperature water typically enters the water heater tank at or near the bottom of the water heater tank, and “pushes out” or supplies the hotter water at or near the top of the water heater tank to service the hot water request. Eventually, the fresh, cold or ambient temperature water entering the bottom of the water heater tank causes the temperature of the water at the bottom of the water heater tank to drop below the water temperature set point. The sensor detects when this occurs, and the controller typically turns on and repeats the heat cycling described above to maintain a water temperature at or near the water temperature set point.
Under certain condition, such as when short water draws occur, the water temperature near the top of the water heater tank can increase well above the water temperature set point, which is sometimes referred to as “stacking”. “Stacking” occurs when hotter water located at the top of the water heater tank become “stacked” on top of colder water at the bottom of the tank. In some cases, the water at or near the top of the water heater tank may become substantially hotter than the lower, safer temperature water at or near the bottom of the water heater tank.
Water heaters with a single temperature sensor located at or near the bottom of the water heater tank may be particularly susceptible to stacking. The temperature sensor may sense the water temperature at or near the bottom of the water heater tank, which during stacking is less than the water temperature set point, even though the water at or near the top of the water heater tank may be substantially hotter than the water temperature set point. In response, the controller often turns on the heating element of the water heater. Because only a relatively small amount of colder water may be located at or near the bottom of the water heater tank, particularly in short water draw situations, the resulting heating cycles may be relatively short. Also, the heating cycles may occur relatively often. This operation can create and/or exacerbate water “stacking” in the water heater, which in many cases, is undesirable.